The present application relates to the art of magnetic resonance imaging. It finds particular application in conjunction with whole body resonators for medium and high field, particularly 1.5 Telsa, magnetic resonance imagers and will be described with particular reference thereto. It is to be appreciated, however, the invention may find broader application with resonators for spectrometers, with imagers adapted to other body parts, with higher and lower magnetic field strengths, and the like.
Heretofore, whole body magnetic resonance imaging apparatus have utilized a radio frequency resonator coil for inducing magnetic resonance in a region of the body of interest. Traditionally, the resonator included either distributed phase or saddle coils. Exemplary distributed phase coils and saddle coils are described in U.S. Pat. No. 4,439,733, issued Mar. 27, 1984 to Waldo S. Hinshaw, et al.
When performing proton magnetic resonance imaging in the range of 1.0 to 2.0 Tesla, a corresponding operating or resonance frequency of 40 to 90 megahertz is required. It is to be appreciated that the resonance frequency of the prior art coils varies with both their dimension and configuration. In order to attain the greater than 40 megahertz frequencies required for proton imaging at high field strengths, the saddle and distributed phase coils have been combined with distributed capacitance or scaled down to relatively small diameters. The distributed capacitance techniques lowered the coil quality. Reducing the diameter rendered the coil unsuitable for receiving the full body or torso of a human patient.
Other small diameter coil designs have been utilized to achieve resonance frequencies greater than 40 megahertz. However, the resonance frequency drops as these smaller diameter resonators are scaled up to dimensions suitable for receiving the torso of a human patient therein. For example, "Slotted Tube Resonator: A New NMR Probe Heat at High Observing Frequencies" by H. J. Schneider and P. Dullenkopf, Rev. Sci. Inst., 48, No. 1, 68-73 Jan. (1977) discloses a slotted tube resonator which is about 10 mm in diameter. Because the length to diameter ratio is in excess of 10 to 1, such a coil is not amenable to enlargement to whole body diameters. Schneider and Dullenkopf also disclose a crossed, slotted tube resonator in "Crossed Slotted Tube Resonator (CSTR) A New Double Resonance NMR Probe Head", Rev. Sci. Inst., 48, No. 7, 832-834 July (1977). A further description of magnetic resonance resonators is provided by D. I. Hoult in "The NMR Receiver: A Description and Analysis of Design", Progress in NMR Spectroscopy, 12, 41-77 (1978). The Hoult article discloses a coil which has a high frequency but which design is non-symmetric and unbalanced. A variation on these coils is shown by D. W. Alderman and D. M. Grant in "An Efficient Decoupler Coil Design Which Reduces Heating In Conductive Samples in Super-conducting Spectrometers", J. Mag. Res. 36, 447-451 (1979). A. Leroy-Willig, et al. in "The Slotted Cylinder: An probe for NMR Imaging", (1984) disclose a configuration which converts the quarter wave length Alderman and Grant slotted tube resonator into a half wave length coil. Another half wave length resonator is disclosed by P. Roschman, "Ring Resonator for RF Probes for Proton Imaging Above 1 Tesla", Abstracts of S.M.R.M. 3rd Annual Meeting, 634-5 (1984).
In accordance with the present invention, a new and improved resonator is provided which can be constructed in sizes sufficiently larger to receive a human patient's torso, yet resonate at frequencies in excess of 40 megahertz.